.ADF file import in ARIS Architect fails sometimes

I have a bunch of .ADF files made by colleagues i ARIS Express. Some are imported in my ARIS Architect v98.1.0 without a hitch and some fail with error message: "<filename> is not a valid ADF model". These files that fail to import are read (and written) without problems by ARIS express v2.4b as are the ones imported without errors.

Any idea on how to find out what is wrong with the failing .ADF models? Any logs to look at for more detailed error messages?

Any specific model construct that is allowed in ARIS Express but not in ARIS Architect?

--Carl